The invention relates to a sensor for detecting particles carried by a fluid flow. Detection of particles in a fluid flowing through a conduit is often desirable, in particular when monitoring flows of well fluids such as water and/or oil and/or gas being produced from underground permeable formation layers that are penetrated by wells or boreholes extending from the surface of the earth into the said layers.
The fluids passing out of the formation layers into the wells are transported through the wells to the surface. Often, these fluids carry solid particles such as sand grains that are dragged from the formation layers by the flowing fluid. These particles will damage the production equipment, such as by erosive action, or settle down in parts of the equipment, thereby frequently necessitating cleaning of the equipment. To prevent this, the fluid flow should be monitored in order to allow the operator to take the necessary steps when excessive amounts of sand grains are found to be present in the fluid flow.
A system for monitoring the fluid flow carrying particles has included a sensor in the form of a metal rod that is exposed to the flow of fluid. The particles on impinging against the rod, generate acoustic energy, which energy is detected by an acoustical transducer means (such as a piezo-electric element) cooperating with the rod. The transducer means generates electric signals that are representative of the acoustic energy generated by the particles that have contacted the rod. By gauging or calibrating the system, reference data can be obtained that are representative for the total amount and/or mass of particles that is present in the flow.
Another known sensor for detecting particles in a fluid flow is a liquid filled hollow cylindrical metal body having a piezo-electric element freely suspended in the liquid. The impacts made by the particles on the exterior of the metal wall of the body travel through the liquid in the form of pressure waves to the piezo-electric element, and are subsequently transduced into electric signals that are representative of the acoustic energy generated by the impacts.
The known sensors--although suitable for use under many conditions--are not attractive when impacts of said grains or other particles should be measured at specific areas, or locations, in particular not when these areas or locations are of a small size, or when the sensors are to be used for preferentially detecting those particles that arrive from a predetermined direction with respect to the sensor.